In skeletal muscle fibers an action potential or a steady depolarization over the mechanical threshold is followed after a few milliseconds by either a twitch or prolonged contraction. Some of the steps in the chain of events linking the two processes (E-C coupling) are partially understood, others are still unknown. It is known that the transverse tubular system (T system) constitutes the means of carrying the excitatory information from the surface membrane towards the center of the fibres. That this signal triggers the release of Ca ion from the lateral sacs of the sarcoplasmic reticulm (SR) and that the Ca ion activates the contractile apparatus by binding to troponin a protein that inhibits contraction in the absence of Ca ion. Eventually Ca ion is taken back by a calcium pump system into the sarcoplasmic reticulum, bringing about relaxation. This is a brief summary of the salient features in E-C coupling. There are many steps in the E-C chain of events which are not known or are poorly understood. The proposed plan I want to undertake here will deal with some of the E-C coupling steps which are not well defined yet. These are, briefly, the nature and physiological characteristics of the signal which spreads along the tubular membranes during the activation, and the mechanism of the link which couples the depolarization of the T-system to the SR, the process that brings about the release of Ca ion from the SR and some control mechanisms that determine the level of Ca ion within the SR and therefore the amount of released Ca ion and of tension development. Measurements will be made of various electrical, optical, morphological and mechanical properties of muscle cells. Attempts will be made to relate these features and define the physical and biological factors involved in the above process.